Basil Copper – The Great White Space (Sphere, 1978)

Terry Oakes
Blurb:
Beyond the hideous Plain of darkness, past the terrifying secret city, deep within earth’s dank uncharted caverns, a monstrous hybrid race stood guard at the entrance to the great white space.
So it was that the Great Northern Expedition ventured into the horrors of a stupendously vast underground terrain, in search of the legendary opening to another universe, peopled by an unimaginable spawn of darkness…
Thanks to Calenture for the cover scan and blurb.